A fire alarm system using loop-signal lines has been well known. In this type of fire alarm system, fire receivers are connected with a plurality of terminal devices through loop-type signal lines. Thus, even if an abnormality such as a disconnection or a short-circuit occurs halfway along the signal line, this system is capable of controlling devices such as fire doors at the time of fire monitoring or during a fire because each terminal device is connected to a fire receiver through a signal line beyond a position where the abnormality occurs.
In such a conventional fire alarm system, signals can be transmitted between the fire receiver and the terminal devices by making polling signals flow to the loop-signal line unidirectionally at a normal operating time. When any terminal device does not respond, the conventional system determines that terminal devices located after the non-responding terminal device cannot be polled due to a disconnection or a short-circuit, and calls the non-responding terminal device by supplying a polling signal to the loop-signal line in the opposite direction. Consequently, an abnormality generated in the loop-signal line and the generating position are detected.
However, this conventional system takes a long time to detect an abnormality, because, when any terminal device does not respond to a unidirectional polling, the device must be polled in the opposite direction to detect an abnormality in the loop-signal line and the position where the abnormality occurs.
An object of the present invention is to provide a fire alarm system which is capable of detecting an abnormality immediately when an abnormality such as a disconnection occurs in the loop-signal line.